The combustion chamber walls in a cylinder head casting are highly stressed during engine operation. High strength material is needed in this area to obtain long life for the component. While alloy selection and heat treatment play an important role in the final strength of the alloy, the conditions during solidification play an equal role. The rate of solidification of the combustion chamber walls is determined by the wall design, mold materials, core materials, cooling design and process variables. The balance between these variables and the alloy used can be difficult to optimize for highest strength.
One of the process variables that must be balanced is mold wall temperature. If the mold wall that forms the combustion chamber is cold, that will increase the solidification rate, but it can be detrimental to the filling of the mold cavity. Excessive loss of metal temperature during mold filling will cause cold shut defects and contribute to sub-surface porosity. A hot mold will minimize the temperature loss of the liquid metal, but it will also lengthen the solidification time of the casting and increase the microstructure size of the combustion chamber wall material. To achieve a hot mold during filling and a cold mold during solidification, mold cooling chambers for the combustion chamber casting walls are typically activated after the mold filling event. To maximize the solidification rate of the casting, maximum high heat flux from the cooling chambers is desired. The design of the mold cooling chamber which forms the combustion chamber casting walls is important in achieving this maximum heat flux during solidification.
A typical measure of microstructure size in aluminum silicon or aluminum copper cast alloys is secondary dendrite arm spacing (SDAS). This measured length is taken from a cut specimen in the combustion chamber wall. A typical SDAS specification is 25 microns maximum in the bridge wall for a high output engine cylinder head. This microstructure length is desirable across the entire combustion chamber face, but is not obtainable with the conventional process.
A conventional semi-permanent mold assembly for an aluminum alloy cylinder head has water cooling chambers below each of the combustion chamber casting walls. The combustion chamber features and cooling lines are typically made with individual tools which insert into the larger base mold. These inserts are precisely located and secured to the base mold from below, typically with a location dowel pin and four bolt bosses. The cooling line input and exit tubing are also connected from below. The cooling chamber needs clearance from these features, which severely restricts its size.
FIGS. 1-2 show one example of a typical combustion chamber cooling insert 10. FIG. 1 illustrates the internal geometry. The cooling insert 10 is typically made of H13 steel. The upper surface forms the casting surface 15. There is a coolant cavity 20 with a coolant inlet 25 and a coolant outlet 30. There is a baffle 35 which directs the coolant flow from the coolant inlet 25 to the coolant outlet 30 toward the top surface of the coolant cavity 20. FIG. 2 shows the bottom of the combustion chamber insert 10 with the four bolt bosses 40 and the location dowel pin 45.
The space requirements for the bolt bosses 40 and location dowel pin 45 restricts the space for the cooling chamber diameter itself. This requires a wall thickness of about 25 mm (or 50 mm total wall thickness). As a result, a combustion chamber insert with a total diameter of 75 mm has a typical coolant cavity diameter of only about 25 mm, an 85 mm insert has coolant cavity of about 35 mm, a 95 mm insert has a coolant cavity of about 45 mm, and a 105 mm insert has a coolant cavity of about 55 mm. Consequently, the cooling requirements for a SDAS of 25 microns or less are difficult to achieve with standard cooling chamber designs. The limited chamber surface area and the mass of steel above the bolt bosses cause a slow thermal response to the casting wall from the activated coolant.